1. Field of the Invention
The present invention relates to apparatus for measuring temperatures, and more particularly to apparatus for temperature measurement of structures placed in extreme environments such as process furnaces for petrochemical applications.
2. The Related Art
Many industrial processes, particularly the petrochemical industry, require accurate measurement of the surface or "skin" temperatures of structures such as tubes or pipes placed within a furnace or other vessel. Such temperature monitoring is necessary to avoid overheating conditions and possible failures. For this purpose, measuring devices such as thermocouples are placed at locations on the pipe or other structure that are expected to encounter the greatest exposure to heat sources, such as from radiation or flame exposure in a furnace. A common problem in such applications is the generally short life expectancy of unprotected thermocouples that are exposed to such temperature extremes.
A further problem relates to the method of attachment of the thermocouples to the pipe or other structure. Generally it is necessary to attach the thermocouple cables to pipe by welding. Thus, replacement of a thermocouple is both time consuming and expensive because the weld must be ground down to remove the thermocouple. Such removal requires a process shutdown to perform, and further requires extreme care to ensure that the pipe is not weakened by penetrating its surface.
In response to these problems, a removable temperature measuring device has been developed. This device is described in U.S. Pat. No. 5,382.093, which is assigned to Gay Engineering & Sales Co., Inc., the owner of the present invention. The device includes a sheathed thermocouple adapted to be removably inserted into the open end of a curved guide tube that carries a positioning pad at its closed end. The pad is welded to the pipe whose temperature is to be measured, and a similarly curved, insulated heat shield is positioned over the guide and the pad, and then also welded to the pipe. In this manner, the thermocouple is easily replaced by pulling it free of the guide tube.
While useful for its intended purpose, the device of the '093 patent requires that the thermocouple be precisely placed within the heat shield for accurate measurement, e.g., of a pipe's skin temperature. In other words, proper thermocouple placement relative to the heat shield is critical for accurate skin temperature measurement. The insulation disclosed in the '093 patent is sold under the trademark KAO-WOOL, and is an easily deformable blanket-like material having a low heat transfer coefficient. Thus, there is no "hard" recess or boundary that dictates proper placement of the heat shield over the welded pad and thermocouple.
Furthermore, the ease with which the KAO-WOOL insulation is compressed makes it possible for the shield to be installed even if excessive weld material has been applied when attaching the thermocouple positioning pad to the structure or pipe. Excessive weld material applied to the positioning pad will likely cause heat to be transferred from the shield to the weld pad, and thereby cause the thermocouple secured by the pad to indicate an artificially high pipe skin temperature.
In response to these problems, it is an object of the present invention to provide a moldable insulating material within a heat shield that can be conformed to the shape of the weld pad and the thermocouple element to ensure that the shield is precisely placed over the pad and thermocouple for proper skin temperature measurement.
It is a further object to provide such a moldable insulating material having sufficient rigidity that the material will not be able to lie flush against the pipe surface if excessive weld material has been applied to the weld/positioning pad.